Previously, wave_reader computed the hill height as a fixed

  zgrid(:,1,1) - MinZ // matlab notation

which implicitly assumes that z increases along the third index.  If
that assumption is violated, the BPE calculation breaks (in ways that
can cause assertion failures based on roundoff error, at that).

Fixing the calculation as min(zgrid(i,1,:)) at initialization time
alleviates this problem.